When working on scripts that will modify the DOM of third party pages you will have to apply custom styles to those elements.
This can cause problems in the host application and you need to be mindful of that.
Rarely use ids.
I’m very tempted to say “NEVER“ use ids, but I know better than to be absolute about these things anymore.
Id’s on elements should be unique per page, so using ids will prevent you from including multiple widgets in the same page.
If you have to use ids, make sure the name is unique. EX: dynamic-prog-widget.
Close your tags.
You never know about the DOCTYPE of the page you will be modifying, so a good rule of thumb is to close all your tags and write HTML that should validate as XHTML transitional.
Your script could check the page DTD and generate HTML accordingly but this will increase the script difficulty considerable.
You can also have several versions of your widget (good luck maintain that), or indicate clearly what HTML version you are compatible with.
We all know that we shouldn’t in-line styles in our pages and always define then in style sheets.
This rule is usually broken when working with embedded widgets. You will see this approach in some of Google widgets, like the embedded search.
The advantage of this approach is that there is no risk of interfering with the styles of the application.
The disadvantage is that it makes life very difficult for the host application to modify the styling of your widget due to the CSS precedence rules.
It also can increase code size significantly if the widget needs to be inserted multiple times in the same page.
“Namespace” your rules.
If you use classes make sure you don’t use the CSS !important rule in any of your classes.
Don’t apply rules to HTML elements or ids (well you shouldn’t be using ids anyway).
Make sure your classes have some unique name, you can use a prefix, like the name of your application. Ex: .dynamic-prog-widget-container
This is a fairly new(est) concern. We are starting to use web-fonts more and more nowadays, specially for icons. Even when the possibility of clashing is minimal you can still override a font name by mistake, so make sure you use similar naming conventions as mentioned in the previous paragraph.
Make sure that links either point to new windows, or you allow for the user to configure this behaviour as wanted.
An easy way to open a new window is using target=”_blank” in your links.
Try to avoid resizing as much as possible. Since you don’t know the code that will surround your widget, resizing it may cause odd behaviour and make for a poor user experience.
And most of all, be mindful of any interaction the user may have with your widget.